Container structures can comprise an oriented thermoplastic polyester resin material. Such resins can be a source of reactive organic materials that can be eluted from the packaging into, for example, a food material held within the container. Such reactive materials, including an aldehyde material, can result in undesirable off-odors or off-flavors in a food, or off-taste in water or beverage drink. The invention relates to polyester pellet or chip coated with active materials that can prevent the formation of or scavenge the organic material during preform and bottle manufacturing methods. The invention further relates to the polyester preform comprising thermoplastic polyester and, dispersed in the thermoplastic resin, an active material that can act to prevent the formation of or scavenge volatile organic components. Lastly, the invention relates to a thermoplastic beverage container and methods of making the chip, preform or container.
Polyethylene terephthalate (PET) packaging materials in the form of film, shaped containers, bottles, etc. have been known. Further, rigid, or semi-rigid, thermoplastic beverage containers have been made from preforms that are in turn molded from pellets or chips etc. Biaxially oriented blow molded thermoformed polyester beverage containers are disclosed in J. Agranoff (Ed) Modern Plastics, Encyclopedia, Vol. 16, No. 10A, P. (84) pp. 192-194. These beverage containers are typically made from a polyester, a product of a condensation polymerization. The polyester is typically made by reacting a dihydroxy compound and a diacid compound in a condensation reaction with a metallic catalyst. Dihydroxy compounds such as ethylene glycol, 1,4-butane diol, 1,4-cyclohexane diol and other diol can be copolymerized with an organic diacid compound or lower diester thereof such diacid. Such diacidic reactants include terephthalic acid, 2,6-naphthalene dicarboxylic acid, methyl diester thereof, etc. The condensation/polymerization reaction occurs between the dicarboxylic acid, or a dimethyl ester thereof and the glycol material in a heat driven metal catalyzed reaction that releases water or methanol as a reaction by-product leaving, a high molecular weight polyester material. Bulk resin is formed as a convenient flake, chip or pellet adapted for future thermal processing. Bulk polyester material can be injection blow molded directly into a container. Alternately, the polyester can be formed into an intermediate preform that can then be introduced into a blow-molding machine. The polyester is heated and blown to an appropriate shape and volume for a beverage container. The preform can be a single layer material, a bilayer or a multilayer preform.
Metallic catalysts are used to promote a polymerization reaction between diacid material and the dihydroxy compound. At the beginning of the melt phase, ethylene glycol, terephthalic acid, or ester thereof, and metallic catalysts are added to the reactor vessel. Various catalysts are known in the art to be suitable for the transesterification step. Salts of organic acids with bivalent metals (e.g. manganese, zinc, cobalt or calcium acetate) are preferably used asxe2x80x94direct esterification or trans-esterification catalysts, which in themselves also catalyze the polycondensation reaction. Antimony, germanium and titanium compounds are preferably used as polycondensate catalysts. Catalysts that may be used include organic and inorganic compounds of one or more metals alone or in combination with the above-described antimony, also including germanium and titanium. Suitable forms of antimony can be used, including inorganic antimony oxides, and organic compounds of antimony, such as antimony acetate, antimony oxalate, antimony glycoxide, antimony butoxide, and antimony dibutoxide. Antimony-containing compounds are currently in widespread commercial use as catalysts that provide a desirable combination of high reaction rate and low color formation. Titanium may be chosen from the group consisting of the following organic titanates and titanium complexes: titanium oxalate, titanium acetate, titanium butylate, titanium benzoate, titanium isoproprylate, and potassium titanyl oxalate. Organic titanates are not generally used in commercial production. At the end of the melt phase, after polymerization is complete and molecular weight is maximized, the product is pelletized. The pellets are treated in solid-state polycondensation to increase intrinsic viscosity in order to obtain bottle resin of sufficient strength. The catalysts typically comprise metallic divalent or trivalent cations. The treatment of polyester materials containing such catalysts can result in byproduct formation. Such byproduct can comprise reactive organic materials such as an aldehyde material, commonly analyzed as acetaldehyde. The formation of acetaldehyde materials can cause off odor or off taste in the beverage and can provide a yellowish cast to the plastic at high concentrations. Polyester manufacturers have added phosphorus-based additives as metal stabilizers to reduce acetaldehyde formation. Many attempts to reduce aldehyde formation have also caused problems. Antimony present as Sb+1, Sb+2 and Sb+3 in the polyester as catalyst residues from manufacture can be reduced to antimony metal, Sb0, by the additives used to prevent aldehyde formation or scavenge such materials. Formation of metallic antimony can cause a gray or black appearance to the plastic from the dispersed, finely divided metallic residue.
The high molecular weight thermoplastic polyester can contain a large variety of relatively low molecular weight compound, (i.e.) a molecular weight substantially less than 500 grams per mole as a result of the catalytic mechanism discussed above or from other sources. These compounds can be extractable into food, water or the beverage within the container. These beverage extractable materials typically comprise impurities in feed streams of the diol or diacid used in making the polyester. Further, the extractable materials can comprise by-products of the polymerization reaction, the preform molding process or the thermoforming blow molding process. The extractable materials can comprise reaction byproduct materials including formaldehyde, formic acid, acetaldehyde, acetic acid, 1,4-dioxane, 2-methyl-1,3-dioxolane, and other organic reactive aldehyde, ketone and acid products. Further, the extractable materials can contain residual diester, diol or diacid materials including methanol, ethylene glycol, terephthalic acid, dimethyl terephthalic, 2,6-naphthalene dicarboxylic acid and esters or ethers thereof. Relatively low molecular weight (compared to the polyester resin) oligomeric linear or cyclic diesters, triesters or higher esters made by reacting one mole of ethylene glycol with one mole of terephthalic acid may be present. These relatively low molecular oligomers can comprise two or more moles of diol combined with two or more moles of diacid. Schiono, Journal of Polymer Science: Polymer Chemistry Edition, Vol. 17, pp. 4123-4127 (1979), John Wiley and Sons, Inc. discusses the separation and identification of PET impurities comprising poly(ethylene terephthalate) oligomers by gel permeation chromatography. Bartl et al., xe2x80x9cSupercritical Fluid Extraction and Chromatography for the Determination of Oligomers and Poly(ethylene terephthalate) Filmsxe2x80x9d, Analytical Chemistry, Vol. 63, No. 20, Oct. 15, 1991, pp. 2371-2377, discusses experimental supercritical fluid procedures for separation and identification of a lower oligomer impurity from polyethylene terephthalate films.
Foods or beverages containing these soluble/extractables derived from the container, can have a perceived off-taste, a changed taste or even, in some cases, reduced taste when consumed by a sensitive consumer. The extractable compounds can add to or interfere with the perception of either an aroma note or a flavor note from the beverage material. Additionally, some substantial concern exists with respect to the toxicity or carcinogenicity of any organic material that can be extracted into beverages for human consumption.
The technology relating to compositions used in the manufacture of beverage containers is rich and varied. In large part, the technology is related to coated and uncoated polyolefin containers and to coated and uncoated polyester that reduce the permeability of gasses such as carbon dioxide and oxygen, thus increasing shelf life. The art also relates to manufacturing methods and to bottle shape and bottom configuration. Deaf et al., U.S. Pat. No. 5,330,808 teaches the addition of a fluoroelastomer to a polyolefin bottle to introduce a glossy surface onto the bottle. Visioli et al., U.S. Pat. No. 5,350,788 teaches methods for reducing odors in recycled plastics. Visioli et al. disclose the use of nitrogen compounds including polyalkylenimine and polyethylenimine to act as odor scavengers in polyethylene materials containing a large proportion of recycled polymer.
Wyeth et al., U.S. Pat. No. 3,733,309 show a blow molding machine that forms a layer of polyester that is blown in a blow mold. Addleman, U.S. Pat. No. 4,127,633 teaches polyethylene terephthalate preforms which are heated and coated with a polyvinylidene chloride copolymer latex that forms a vapor or gas barrier. Halek et al., U.S. Pat. No. 4,223,128 teaches a process for preparing polyethylene terephthalate polymers useful in beverage containers. Bonnebat et al., U.S. Pat. No. 4,385,089 teaches a process for preparing biaxially oriented, hollow thermoplastic shaped articles in bottles using a biaxial draw and blow molding technique. A preform is blow molded and then maintained in contact with hot walls of a mold to at least partially reduce internal residual stresses in the preform. The preform can be cooled and then blown to the proper size in a second blow molding operation. Gartland et al., U.S. Pat. No. 4,463,121 teaches a polyethylene terephthalate polyolefin alloy having increased impact resistance, high temperature, dimensional stability and improved mold release. Ryder, U.S. Pat. No. 4,473,515 teaches an improved injection blow molding apparatus and method. In the method, a parison or preform is formed on a cooled rod from hot thermoplastic material. The preform is cooled and then transformed to a blow molding position. The parison is then stretched, biaxially oriented, cooled and removed from the device. Nilsson, U.S. Pat. No. 4,381,277 teaches a method for manufacturing a thermoplastic container comprising a laminated thermoplastic film from a preform. The preform has a thermoplastic layer and a barrier layer which is sufficiently transformed from a preformed shape and formed to a container. Jakobsen et al., U.S. Pat. No. 4,374,878 teaches a tubular preform used to produce a container. The preform is converted into a bottle. Motill, U.S. Pat. No. 4,368,825; Howard Jr., U.S. Pat. No. 4,850,494; Chang, U.S. Pat. No. 4,342,398; Beck, U.S. Pat. No. 4,780,257; Krishnakumar et al., U.S. Pat. No. 4,334,627; Snyder et al., U.S. Pat. No. 4,318,489; and Krishnakumar et al., U.S. Pat. No. 4,108,324 each teach plastic containers or bottles having preferred shapes or self-supporting bottom configurations. Hirata, U.S. Pat. No. 4,370,368 teaches a plastic bottle comprising a thermoplastic comprising vinylidene chloride and an acrylic monomer and other vinyl monomers to obtain improved oxygen, moisture or water vapor barrier properties. The bottle can be made by casting an aqueous latex in a bottle mold, drying the cast latex or coating a preform with the aqueous latex prior to bottle formation. Kuhfuss et al., U.S. Pat. No. 4,459,400 teaches a poly(ester-amid) composition useful in a variety of applications including packaging materials. Maruhashi et al., U.S. Pat. No. 4,393,106 teaches laminated or plastic containers and methods for manufacturing the container. The laminate comprises a moldable plastic material in a coating layer. Smith et al., U.S. Pat. No. 4,482,586 teaches a multilayer of polyester article having good oxygen and carbon dioxide barrier properties containing a polyisophthalate polymer. Walles, U.S. Pat. Nos. 3,740,258 and 4,615,914 teaches that plastic containers can be treated, to improve barrier properties to the passage of organic materials and gases, such as oxygen, by sulfonation of the plastic. Rule et al., U.S. Pat. No. 6,274,212 teaches scavenging acetaldehyde using scavenging compounds having adjacent to heteroatoms containing functional groups that can form five or six member bridge through condensation with acetaldehyde. Al-Malaika PCT WO 2000/66659 and Weigner et al., PCT WO 2001/00724 teach the use of polyol materials as acetaldehyde scavengers. Wood, et al . U.S. Pat. Nos. 5,837,339, 5,883,161 and 6,136,354, teach the use of substituted cyclodextrin in polyester for barrier properties.
Further, we are aware that the polyester has been developed and formulated to have high burst resistance to resist pressure exerted on the walls of the container by carbonated beverages. Further, some substantial work has been done to improve the resistance of the polyester material to stress cracking during manufacturing, filling and storage.
Beverage manufacturers have long searched for improved barrier material. In larger part, this research effort was directed to carbon dioxide (CO2) barriers, oxygen (O2) barriers and water vapor (H2O) barriers. More recently, original bottle manufacturers have had a significant increase in sensitivity to the presence of beverage extractable or beverage soluble materials in the resin or container. This work has been to improve the bulk plastic with polymer coatings or polymer laminates of less permeable polymer to decrease permeability. However, we are unaware of any attempt at introducing into bulk polymer resin or polyester material of a beverage container, an active complexing compound to scavenge metal catalyst residues contained in the polyester resin during the preform manufacturing process, reducing catalytically generated beverage extractable or beverage soluble material caused by catalyst residues in the resin or container.
Even with this substantial body of technology, substantial need has arisen to develop biaxially oriented thermoplastic polymer materials for beverage containers that can substantially reduce the elution of reactive organic materials into a food or beverage in the container or reduce the passage of permeants in the extractable materials that pass into beverages intended for human consumption.
Stabilization of polyester resins and absorption of reactive organics such as acetaldehyde have drawn significant attention. Proposals for resolving the problem have been posed. One proposal involves using active stabilizers including phosphor compounds and nitrogen heterocycles as shown in WO 9744376, EP 26713 and U.S. Pat. No. 5,874,517 and JP 57049620. Another proposal, which has obtained great attention, includes solid state polycondensation (SSP) processing. The materials after the second polymerization stage are treated with water or aliphatic alcohols to reduce residuals by decomposition. Lastly, acetaldehyde can be scavenged with reactive chemical materials including low molecular weight partially aromatic polyamides based on xylylene diamine materials and low molecular weight aliphatic polyamides. [See, U.S. Pat. No. 5,258,233; 6,042,908 and European Patent No. 0 714 832, commercial polyamides see WO 9701427, polyethylene imine see 5,362784, polyamides of terephthalic acid see WO 9728218 and the use of inorganic absorbents such as zeolytes, see U.S. Pat. No. 4,391,971.]
Bagrodia, U.S. Pat. No. 6,042,908 uses polyester/polyamide blends to improve flavor of ozonated water. Hallock, U.S. Pat. No. 6,007,885 teaches oxygen-scavenging compositions in polymer materials. Ebner, U.S. Pat. No. 5,977,212 also teaches oxygen-scavenging materials in polymers. Rooney, U.S. Pat. No. 5,958,254 teaches oxygen scavengers without transitional catalysts for polymer materials. Speer, U.S. Pat. No. 5,942,297 teaches broad product absorbance to be combined with oxygen scavengers in polymer systems. Palomo, U.S. Pat. No. 5,814,714 teaches blended mono-olefin/polyene interpolymers. Lastly, Visioli, U.S. Pat. No. 5,350,788 teaches method for reducing odors in recycled plastics.
In implementing the technologies using various scavenging materials in polyester beverage polymers, a significant need remains for technology that reduces the concentration of organic materials such as aldehyde, ketone and acids in polyester without the reduction of antimony to gray or black metallic residue. In particular, a reduction in acetaldehyde residues in polyester is required. Further, a need exists to obtain reduced acetaldehyde concentration in polyesters along with introducing barrier properties in the polyester material.
We have found that polyester resin and polyester beverage containers can be made with an active component that can act to inhibit reactive organic chemical compound formation. The active components also offer an organic vapor barrier property to the container material. We have found that a small amount of a specific substituted cyclodextrin compound can be coated onto the polyester chip or pellet during bulk polyester resin manufacture. The polyester chip with the cyclodextrin compound can then be introduced into an extruder for the purpose of injection molding a polyester preform article or directly blowing the bottle. During extrusion, the cyclodextrin compound mixes with the melt polymer at high temperature during a set residence time. At the temperature of the melt extrusion, the cyclodextrin compound reacts with, complexes or associates with the metallic catalyst residues and prevents the production of catalytically generated reactive organic compounds, including aldehyde materials such as acetaldehyde. The cyclodextrin compound can also react with and scavenge volatile reactive materials such as acetaldehyde formed during the melt process. A preform or blow molding residence time is selected that results in effective aldehyde concentration reduction but without cyclodextrin or polymer degradation. Such a reduction in aldehyde concentration reduces or eliminates major off-odors and off-flavors in the thermoplastic polymer.
We have found that a small, but critical, loading of a specific cyclodextrin material on the thermoplastic polymer obtains excellent scavenging and barrier properties. Preferably, the cyclodextrin is formed in a coating layer on the polyester chip or pellet. Such coatings are made by dispersing or dissolving the cyclodextrin compound typically in solvent preferably in an aqueous solution and dispersing or spraying such aqueous solution onto the polymer chip or pellet following polycondensation and preferably after SSP. This amount of cyclodextrin is sufficient to provide such properties without unacceptable commercial discoloration of the polymer resin or any reduction in polymer clarity or physical properties. The cyclodextrin compound is typically incorporated with, dispersed into or suspended in the bulk polymer material used to make the beverage container. We have also found that the purity of the cyclodextrin aqueous solution is important in achieving reduced aldehyde, reduced color formation and preventing antimony reduction. Once formed, an aqueous cyclodextrin solution can be purified by contacting the solution with an activated charcoal absorbent, an ion exchange resin or a filtration apparatus including nanofiltration, reverse osmosis, etc. equipment.
Preferably, the cyclodextrin compound utilized in the technology of the invention involves a substituted xcex2- or xcex1-cyclodextrin. Preferred cyclodextrin materials are substituted on at least one of the 3-OH of the glucose moiety in the cyclodextrin ring. xcex2-Cyclodextrin materials comprise seven glucose moieties forming the cyclodextrin ring. Any of such hydroxyl groups can be substituted. The degree of substitution (D.S.) of the cyclodextrin material can range from about 0.3 to 1.8; preferably the degree of substitution can range from about 0.5 to 1.2. We found that complexing metallic catalyst residues in the polymer material, a beta or alpha cyclodextrin is preferred. Further, the degree of substitution has an important role in ensuring that the cyclodextrin is compatible with the melt polymer, but is not so substituted that the cyclodextrin cannot participate in complexing catalyst residues. We have further found that the amount of the substituted cyclodextrin material useful in preventing the formation of aldehyde by complexing metallic catalyst residues is less than the amount of cyclodextrin active in barrier structures. The effective amount of a substituted cyclodextrin for aldehyde suppression ranges from about 100 ppm to 1,400 ppm based on the polymer composition as a whole preferably 350 ppm to 900 ppm. The principle mechanistic action of the substituted cyclodextrin material is a coordination complex of the metallic catalyst where more than one metal ion is bound per cyclodextrin. Metallocyclodextrins are formed from substituted cyclodextrins (6-position xe2x80x94OH) which consist of two cyclodextrins linked together through the secondary hydroxyl groups (3- and 2-positions) of the unmodified (native) cyclodextrin losing a proton to produce an alkoxide to coordinate a metal ion forming the simplest type of metallocyclodextrin. Accordingly, a substantial and effective fraction of the cyclodextrin must be available for catalyst residue complexation to accomplish the goal of the invention. The compatible cyclodextrin compounds are introduced into the melt thermoplastic substantially free of an inclusion complex or inclusion compound. For this invention the term xe2x80x9csubstantially free of an inclusion complexxe2x80x9d means that the quantity of dispersed cyclodextrin material in the coating on the polyester chip or pellet is free of a complex material or xe2x80x9cguest compoundxe2x80x9d in the central pore of the cyclodextrin molecule. A first aspect of the invention comprises a thermoplastic pellet or chip having a major proportion of the thermoplastic polyester material used in making the preform or the beverage container. The pellet or chip comprises an exterior coating layer, an effective metal catalyst scavenger and volatile organic barrier-providing amount of a cyclodextrin compound. Such an exterior coating of cyclodextrin can be made from an aqueous solution of the cyclodextrin material. The aqueous solution can be made by dissolving a cyclodextrin material in an aqueous medium to form a solution and purifying the solution. A second aspect of the invention comprises a process of forming a purified cyclodextrin solution by contacting a cyclodextrin solution with and activated carbon absorbent, an ion exchange resin, or membrane filtration equipment. A third aspect of the invention comprises a thermoplastic preform having within the polymer matrix, an effective amount of the cyclodextrin compound for reducing volatile organic materials such as acetaldehyde produced during injection molding and for introducing a barrier property into the thermoplastic polymer. A fourth aspect of the invention comprises a thermoplastic beverage container having the metal catalyst scavenger property and a volatile organic barrier property that results from the manufacture of the beverage container from the preform of the invention. Lastly, a fifth aspect of the invention comprises a method for manufacturing a polyester beverage container from the coated pellet or chip of the invention through a preform stage. In each of these aspects, the use of the purified cyclodextrin material results in a clear, substantially water white polyester material having little or no organic material to produce off odors or off flavors in the food material within a polyester container.